1. Field of the Invention
The present invention relates to voice coil assemblies for use in loudspeakers or other apparatus which transform electrical signals into mechanical motion and more particularly to a voice coil assembly which includes a ceramic voice coil form and transducer windings formed by a molybdenum-manganese metallization onto the voice coil form.
2. Description of the Prior Art
Presently a voice coil assembly includes a voice coil form which is generally formed out of either thin paper or thin aluminum. The voice coil assembly also includes at least one insulated wire which is generally formed out of either copper with an enameled coating or aluminum with a thin anodized coating and which is wound onto the voice coil form. The insulated wire may be in the form of a ribbon to increase the winding efficiency. Adhesives which are either one of the many high temperature epoxies or clear enamel coatings are used to secure the windings in place after the insulated wire is wound onto the voice coil form.
U.S. Pat. No. 3,935,402, entitled Loudspeaker Voice Coil Arrangement, issued to Martin Gersten, on Jan. 27, 1976, teaches a loudspeaker voice coil assembly which has improved power handling capability. The loudspeaker voice coil assembly is wound of rectangular cross-section aluminum wire having a flexible anodized coating. A thin aluminum cylindrical voice coil form having an anodized coating is adhered to the windings of the loudspeaker voice coil assembly with a thin coating of a cement. The anodized coating on the aluminum wire serves not only to electrically insulate the turns one from another and from the aluminum, heat-radiating thin aluminum voice coil form, but also serves to enhance the efficiency of the cement bonding. There is a reliability problem in this type of voice coil assembly because it is difficult to lead/tin solder aluminum wire even when using aluminum fluxes for which there are no known Military specifications. In static tests performed on a work bench in open air, no magnetic loading or mass to accelerate or decelerate these loudspeaker voice coil assemblies have continuously dissipated 150 watts rms audio power. In dynamic tests of this type of voice coil assembly wherein not only is the voice coil form heated to a reasonable operating temperature, but is also subjected to a high energy, low frequency transient signal to its transducers windings, the voice coil assembly will blow up.
U.S. Pat. No. 4,376,233, entitled Securing of Lead Wires to Electro-Acoustic Transducers, issued to Yoshiyuki Kamon and Yoshihiro Yokoyamo on Mar. 8, 1983, teaches an electro-accoustic transducer of the dynamic type, particularly a small loudspeaker or microphone, such as a loudspeaker for use in headphones includes a magnetic circuit including an air gap, a diaphragm having a voice coil assembly disposed in the air gap, and lead wires for the voice coil assembly with the lead wires extending substantially tangentially from the voice coil and being bonded to the diaphragm by two different kinds of adhesive, a relatively hard adhesive being used near the voice coil and a relatively soft adhesive being used near the periphery of the diaphragm.
U.S. Pat. No. 4,322,583, entitled Voice Coil Bobbin Connection to Loudspeaker Diaphragm of Honeycomb Core Sandwiched by Sheets, issued to Keijiro Maeda on Mar. 30, 1982 teaches a diaphragm for a flat electroacoustical transducer such as a loudspeaker which includes first and second sheet members having a honeycombed core structure sandwiched therebetween each and includes an interconnection between each diaphragm and the voice coil form to avoid shifts and delays in transmitting vibration to each diaphragm and eliminate the tendency to introduce extraneous auditory sound.
U.S. Pat. No. 4,322,584, entitled Voice Coil Bobbin for Planar Diaphragm, issued to Kunihiko Shimade and Yukio Tsuchiya on Mar. 30, 1982 teaches a voice coil form for construction which includes plural voice coil form frames which are joined and bonded together to form the composite voice coil form.
U.S. Pat. No. 3,358,088, entitled Electromechanical Transducer, issued to Robert A. Gault on Dec. 12, 1967, teaches an electromechanical transducer which has a thermally conductive material which is bonded in close thermal proximity to the transducer windings of the voice coil assembly in order to increase the rate of heat dissipation of heat from the transducer windings thereby appreciately increasing the wattage rating of the voice coil assembly.
U.S. Pat. No. 3,656,015, entitled Combined Linear Motor and Carriage, issued to Donald E. Gillum on Apr. 11, 1972, teaches a voice coil linear motor.
U.S. Pat. No. 4,270,073, entitled Position Control in Disk Drive System, issued to Jefferson H. Harman on May 26, 1981, teaches a voice coil type linear motor controls the movement and positioning of a transducer which cooperates with a spinning disc.
U.S. Pat. No. 4,149,201, entitled Transducer Centering System, issued to Daniel C. Card on Apr. 10, 1979, teaches an improved transducer centering system which includes a voice coil linear motor.
Voice coil assemblies of the types described above are subject to a variety of different kinds of failures. The voice coil form which is formed out of a thin and flexible material may deform from its desired circular geometry and contact the magnet or the pole piece. This contacting will result in distorted audio reproduction and eventually will wear through both the voice coil form and the insulated windings thereby creating a partial electrical short. The use of a stiffer voice coil form eliminates the problem of contacting.
When an amplifier overheats the voice coil assembly it may become shorted because the enamel coating on the copper wire will burn off causing a loss of insulation between the windings resulting in a short circuit. The enameled coating of a conventional insulated copper wire will carbonize or otherwise fail at about 250.degree. C. The use of aluminum wire having an anodized coating rather than the conventional enamel copper wire means that the thermal dissipation is limited only by the melting point of the aluminum wire rather than by the thermal destruction of the enameled coating. The thin anodized coating on either the aluminum wire or the aluminum ribbon has good insulating properties up to the melting point of the aluminum wire before its insulating properties are lost. However, the voice coil assembly which is made with an aluminum wire with a thin anodized coating in the range of one micron is only as good as the epoxy that holds it together. Once the windings break free from the epoxy, rubbing between the windings will occur and easily break down the super thin anodized coating on the aluminum wire thereby resulting in intermittant shorts and/or dead shorts. Each turn of the windings is bonded to each of the adjacent turns of the windings and to the inner cylindrical surface of the voice coil forming a very firm, interdigitated bond. The adhesive is a commercially available epoxy, polyamide cement. There are windings of a voice coil assembly which are capable of dissipating 150 rms audio watts continuously or 250 rms audio watts programmed and which have withstood voice coil assembly operating temperature in excess of 250.degree. C. The dimensional stability of the voice coil assembly is assured from room temperature to 250.degree. C. because both the voice coil winding conductors and the heat dissipating voice coil form are made of the same material and hence have the same coefficent of expansion.
An amplifier of even medium power in the range of 100 watts can overdrive a voice coil assembly having a thin paper voice coil form thereby easily "burning up" the thin paper voice coil form. When the user operates the voice coil assembly at high temperature he may displace windings by softening up the epoxy which secures the windings on the voice coil form so that the epoxy looses its tensile strength thereby allowing the windings to blow off of the voice coil form. The blowing off of the windings will also cause them to come in with either the magnet or the pole piece producing audio distortion. The windings may also break loose from the voice coil form and from the epoxy adhesive thereby allowing the windings to rattle around freely. The rattling of the windings will fatigue the wire of the windings and eventually the voice coil assembly will electrically open up. The insulated wires are attached by conventional soldering using solder with an appropriate flux such as aluminum flux for the aluminum wire. The adhesive material may also be a ceramic cement for attaching the windings to the voice coil form.